Recently, HCCI (Homogeneous-Charge Compression Ignition) combustion in which gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed cylinder has attracted attention. The HCCI combustion is a mode in which the mixture gas combusts at a plurality of positions simultaneously, and thus has a higher combustion speed of the mixture gas than in SI combustion (spark-ignition combustion) which is adopted for general gasoline engines. Therefore, the HCCI combustion is said to be significantly advantageous in terms of thermal efficiency. However, the HCCI combustion has issues such as a combustion start timing of the mixture gas (a timing that the mixture gas self-ignites) greatly varying due to an external factor (e.g., atmospheric temperature) and a control during a transient operation in which an engine load sharply changes is difficult.
Therefore, instead of combusting the entire mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion is referred to as “partial compression-ignition combustion.”
For example, JP2009-108778A discloses an engine adopting a similar concept to the partial compression-ignition combustion. This engine causes flame propagation combustion by spark-igniting a stratified mixture gas which is formed around a spark plug by a supplementary fuel injection, and then performs a main fuel injection inside a cylinder warmed up by an effect of the flame propagation combustion, so as to combust through self-ignition the fuel injected in the main fuel injection.
Here, there is a problem that in the engine adopting the partial compression-ignition combustion, combustion noise becomes loud more easily compared to, for example, a general gasoline engine in which only the SI combustion (spark-ignition combustion) is performed. Thus, during the partial compression-ignition combustion, switching to other combustion modes with different in-cylinder gas state function (e.g., EGR (exhaust gas recirculation) ratio or air-fuel ratio) may be requested according to a factor of combustion noise, etc.
However, if such a switching of the combustion mode according to the factor of the combustion noise, etc. is allowed without any restriction, the switching of the combustion mode may be repeated frequently (e.g., immediately after the switching from one mode to the other mode, resuming to the one mode is performed). In this case, a change in noise (sound) accompanying the switching of the mode occurs frequently, thus a person(s) in a cabin may feel discomfort.